Currently, organic transistors (thin-film transistors) using plastic substrates including polycarbonate and polyethylene terephthalate are under study as flexible devices such as electric paper. However, due to a problem that such plastic substrates slightly expand or contract when heated, improvement in heat resistance properties (low thermal expansion properties) is an urgent issue.
Meanwhile, production processes of an organic thin-film transistor at a lower temperature are under study in order to reduce thermal stress on plastic substrates. Forming and curing a gate insulating film is one of the processes that require highest temperature in producing an organic thin-film transistor. Therefore, a production process of a gate insulating film at a lower temperature has been required.
Developed methods for forming a gate insulating film at a low temperature include a method for anodizing the surface of a gate electrode (see Patent Document 1) and a method including chemical vapor deposition (see Patent Document 2), both of which have complicated production processes.
Therefore, a material that can be easily formed into a film by coating methods such as spin coating or printing is desired. As an example of the production of a gate insulating film by coating, a solution containing poly-4-vinylphenol and poly(melamine-formaldehyde) is applied by spin-coating, followed by curing at 200° C. (see Non-patent Document 1). However, in this method, the treatment temperature is as high as 200° C. and causes a significant influence of the thermal expansion and contraction of a plastic substrate. Therefore, it is difficult to use the method for producing electric paper or the like that is required to have fine pixels.
Soluble polyimides are known as one of the insulating materials that can be produced at a relatively low temperature and expected to have high insulating properties. Since polyimides generally have a high thermal decomposition temperature and a high electric resistance, they are widely used as insulating materials for electronic devices. Disclosed is an example in which a polyimide cured at 180° C. is used as a gate insulating film for a high-precision organic transistor (see Non-patent Document 2).
In recent years, with thin-film transistors having excellent mechanical flexibility, such as organic transistors in particular, electrodes and wirings are formed by irradiation with high-energy ultraviolet rays to reduce production cost (see Non-patent Document 3).
[Patent Document 1]
Japanese Patent Application Publication No. JP-A-2003-258260
[Patent Document 2]
Japanese Patent Application Publication No. JP-A-2004-72049
[Non-patent Document 1]
Journal of Applied Physics (J. Appl. Phys.), Vol. 93, No. 5, 1 Mar. 2003, pp. 2997-2981.
[Non-patent Document 2]
Applied Physics Letters (Appl. Phys. Lett.), Vol. 84, No. 19, 10 May 2004, pp. 3789-3791.
[Non-patent Document 3]
2006 Nen Shu-ki Dai 67 Kai Ouyo-Butsuri Gakkai Gakujutsu Koenkai Kouen Yoko-shu (The Conference Book of the 67th Autumn Meeting of The Japan Society of Applied Physics, 2006), 29 a-ZH-5, 2006, p. 1210.